Communication networks typically include at least one sender and one receiver. In either a wired or wireless network, a signal transmitted between the sender and receiver must be of sufficient magnitude (or strength) to allow the information contained within the signal to be discriminated from the noise which is generally present in the communication network. This may be a greater problem with a wireless network, which typically is more susceptible to noise from various interference sources.
An example of a wireless network is a radio network such as a cellular network commonly utilized for voice and/or data communications between a fixed base station covering a geographic region and mobile devices such as cellular terminals (or phones) present in the covered region. A cellular phone typically includes a radio receiver including an antenna for receiving signals and an amplifier/detector for generating a measure of the strength of received signals or noise. A signal strength measure, commonly known as Radio Signal Strength Indication (RSSI), may be expressed as a logarithmic measure of received signal strength and may be converted to a digital form by an analog to digital converter.
It is known in the prior art that radio signal strength measurements can be useful in determining which base station should serve a cellular phone during a call. In the U.S. AMPS system, the mobile phone would typically use such signal strength measurements to determine the strongest base station to which it should listen for calls during standby (idle) mode. Also in the U.S. AMPS system, base stations belonging to the cellular network typically listen to the signal strengths received from mobile phones that are actively transmitting during calls, and the network uses its measurements to determine an optimum base station for handling a call in progress. When a call in progress is switched from one base station to another, it is commonly known as "handover" or "handoff." Handoffs enable calls to be maintained even though the mobile phone may be changing location.
Cellular phones using a Time Division Multiple Access method conforming to either the European cellular standard known as GSM or any of the American TDMA standards, for example, those known respectively as D-AMPS, IS54, IS136 or PCS1900, may use spare time between transmit and receive timeslots to change frequency and monitor the signal strengths of other base stations. Several measurements of signal strength may be averaged for the same base station. The mobile phone makes measurements of the signal strengths received from surrounding base stations even during the progress of a call. Mobile Assisted Handover (MAHO) may be implemented using these measurements. The averages are typically reported to the currently serving base station, which determines if a handoff should be made to another, base station. The mobile typically reports MAHO RSSI measurements to the network station using a low-bitrate, inband signaling channel called the Slow Associated Control Channel or SACCH. The network uses SACCH measurements to determine the optimum base station to handle a call in progress, preferably the base station that the mobile phone is receiving most strongly.
In order for MAHO to operate effectively, it is preferred that the RSSI measurements that are reported to the network using the SACCH are reasonably accurate over a wide range of signal strengths that may be encountered from base stations. It is known in the prior art to employ logarithmic IF amplifiers with progressive saturation and progressive detectors in order to produce an RSSI signal approximately proportional to the logarithm of the received signal strength. See for example U.S. Pat. Nos. 5,048,059 entitled "Logpolar Signal Processing" and 5,070,303 entitled "Logarithmic Amplifier/Detector Delay Compensation" which are incorporated by referenced herein in their entirety.
Inaccuracy in RSSI measurements may occur when the RSSI value is not exactly proportional to (linearly related to) the received signal strength. Inaccuracy of RSSI measurements may also occur when measuring strong signals that partially saturate amplifing stages prior to the RSSI detectors; a 10 dB increase in received signal level may not then be passed through to the RSSI detectors due to the preceding amplifiers being unable to deliver a 10 dB increase in output signal. The RSSI detectors typically then register a lower increase in signal strength than is actually received. These inaccuracies may vary between mobile phones or change with temperature or other conditions that vary in operation. Therefore, there is a need for an improved means to account for such inaccuracies.